Consistent nondisjunction of B chromosomes in Black Mexican

In reviewing cytological data from crosses in which Black Mexican sweet corn with B chromosomes (BMSC-B) was used as the pollen parent on females lacking B chromosomes, it was observed that the number of B chromosomes in the progeny was almost always even. In fact, of 223 plants from this type of cross, root-tip cytology has revealed that all but four possessed an even number of B chromosomes. Plant RS1044 had 3 B chromosomes and was from a 0B x 9B cross. Plant 86.C10.12 had 9 B's and was from a cross in which the female had no B's and the male had at least 9 B's. Plant 85.103.6 was a 1B/2B chimera (i.e., half 1B and half 2B nuclei in one root-tip) and was from a 0B x 6B cross. Plant 86.C6.11 was from a cross of a female with no B's and a male with at least 7 B's. It was analyzed as having 3 B's, but this was after it had been noticed that most plants from crosses of this type possess an even number of B's. Two more root-tips were analyzed and both were found to have 2 B's.

The cytological data from plants 85.103.6 and 86.C10.12 indicate that B chromosomes may display a low degree of instability in the course of development. That this instability, if it does exist, occurs at a low level is indicated also by comparison of mitotic and meiotic chromosome counts that were performed on 34 plants containing varying numbers of B chromosomes. Of these, only one displayed discordant numbers of B chromosomes in the two analyses. Plant RS650 had 2 B's in root-tip cells and only 1 B in pollen mother cells.

Therefore, it is possible that nondisjunction of all B chromosomes at the second pollen mitosis occurs 100% of the time and that plants containing an odd number of B chromosomes from 0B x B crosses actually represent subsequent instability of B chromosomes in somatic cell divisions. Alternatively, it is possible that, rarely, B chromosomes do undergo normal disjunction in the second pollen mitosis. If this is true, it seems to occur only when the plant possesses a very high number of B chromosomes (at least 9 B's in the two cases presented here).

Thus, it seems quite possible that nondisjunction of B chromosomes at the second pollen mitosis may occur 100% of the time in Black Mexican sweet corn. Utilization of plants with low numbers of B's should virtually guarantee production of male gametes with even numbers of B chromosomes in this inbred line.

In order to study the nature of the influence of genetic background on B chromosome nondisjunction, crosses of BMSC-B to other inbred lines must be made. Preliminary cytological data from A632/BMSC-B hybrids used as pollen sources on 0B plants show 4 progeny with odd B chromosome number out of a total of 21 progeny analyzed. This indicates that the B chromosome nondisjunction rate is a maximum of 81%. Thus, it appears that A632 does not have the proper genetic constitution to support consistent nondisjunction of B chromosomes.

It is interesting to note that early cytological studies of progeny from 0B x B crosses show highly variable amounts of B chromosome nondisjunction (A.E. Longley, J. Agric. Res. 34:769-784, 1927; L.F. Randolph, Genetics 26:608-63 1, 1941; M. Blackwood, Heredity 10:353-366,1956). However, none of these researchers used inbred lines, and in fact often intercrossed stocks to utilize hybrid vigor in maintaining B chromosomes. The only case of 100% nondisjunction reported was found by Randolph when he crossed 0B females and 2B males. In the progeny of this cross he obtained 22 0B, 37 2B, and 2 4B progeny. In all other 0B x B crosses, however, he obtained several progeny with odd B chromosome numbers. His sources of B chromosomes were Black Mexican and Golden Bantam varieties of sweet corn. It is possible that in this particular cross, the 2B plant(s) used as the male was from a Black Mexican inbred line.

It seems quite probable that the genetic constitution of plants containing B chromosomes will have a strong influence on the rate of B chromosome nondisjunction. For example, genes that shorten the duration of the division of the generative nucleus or lengthen the duration of replication of centromeric heterochromatin may have a strong positive influence on B chromosome nondisjunction. At any rate, BMSC-B can be used as a pollen source in crosses with 0B plants to assure nearly all progeny will have even numbers of B's. Likewise, if B-A translocations are maintained in a Black Mexican background, the proportions of hypo- and hyperploid progeny could be maximized.

Rick W Staub and Patricia M. Laurenson
 
 


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